Typically, in motor vehicles using hydrogen to power fuel cells or using hydrogen and compressed natural gas (CNG) to power internal combustion engines, present practice is that fuel is stored in on board tanks maintained at a maximum pressure in the range of about 5000 psi for hydrogen and 3600 psi for CNG. Higher pressures in the range of about 10,000 psi or more are anticipated as the use of hydrogen and CNG becomes more prevalent. I have developed in situ techniques to manage thermal energy differences between high pressure gas in a tank and the environment of the tank; the devices involve heat exchangers fixed within the tank (to absorb and radiate heat) operatively interconnected with an external heat exchanger (correlatively to radiate and absorb heat) in the sequence of the refill and exhaustion of the high pressure gas within the tank. The background of the invention is extensively detailed in my above referenced related application and my previous applications referenced therein. In the specification herein, reference to hydrogen storage tanks correlates with the use of the invention with CNG (compressed natural gas) storage tanks. Although hydrogen is principally referred to in the specification and examples, “hydrogen” is a term in most instances intended to be interchangeable with CNG and both are referred to as a “gas” or “high pressure gas.” Both hydrogen and CNG are high pressure gases with which the invention is useful, typically, but not necessarily, in motor vehicle applications.
Conventionally, high pressure gas vehicle fuel tanks are cylindrical with hemispherical ends and are formed from reinforced fiber composite material (e.g., a carbon fiber exterior winding and an aluminum alloy/plastic interior liner) and, in their various designs, are capable of storing a charge of high pressure gas at an approximately 5000 psi for hydrogen and approximately 3600 psi for CNG, and up to 10,000 psi or more at a maximum rated pressure capacity at a defined temperature. Various designs for high pressure hydrogen refueling stations have been proposed to deal with refueling efficiencies. When the on board fuel tanks of a hydrogen powered vehicle are filled with hydrogen, the pressurized on board gas in the tanks may be characterized as having multiple forms of energy: 1) chemical energy associated with the hydrogen fuel itself (consumed in powering the vehicle), and 2) thermodynamic energy, namely, the mechanical, thermal and internal energy associated with the physics of high pressure refueling of a tank from sources of fuel at the high pressure gas refuel depot.
During a high pressure refueling process involving hydrogen and CNG fueled vehicles, gas within the interiors of the on board storage tanks becomes heated as a result of fuel gas compression when the tank pressure increases and other refueling parameters affect the refill. After refueling, the interior temperature of the gas within the tank and the pressure within the tank both decrease slowly as the fuel gas is consumed during vehicle operation. Conventionally, it is not possible to obtain a full refill tank pressure without pressure compensation during the course of refueling. The charge of fuel pressure input into and stored in the tank must be, at refill (because of the heating compression of the gas), initially in excess of the tank design pressure. Without pressure compensation (an initial overfill), vehicle mileage range is reduced because a full fill is not obtained. When higher optimum tank design pressures are encountered, this condition is exacerbated.
In one response to the overfill dilemma, a slower flow rate may be used during refill, which will result in a lower internal tank temperature, and higher pressure, and increased capacity over time. An undesirable consequence of a slower flow rate during refueling to avoid heat build up is self evident—a longer refueling time. Another solution proposes to cool the station fuel gas before the refueling gas is introduced into the tank; external pre-cooling, however, requires substantial energy, thereby reducing the overall efficiency of a hydrogen/CNG economy. Pre-cooling or other pre treatment of the high pressure refuel gas is generally unnecessary when fill pressures are at 5000 psi or lower, however, as pressures approach or exceed 10,000 psi, cooling (and its opposite, internal tank heating by compression) becomes an important factor in the refueling process.
A pressure overfill as an option likewise requires additional energy expense where additional gas compression is involved and further increases the heat generated in the tank as a result of high pressure compression during the refill process. As tank pressures exceed 3600 psi (for CNG) and 5000 psi and approach or exceed 10,000 psi (for hydrogen), secondary treatment such as cooling becomes an important factor in the refueling process to achieve a full tank capacity fill. When a full fill is achieved, 1) overall vehicle range per each tank refill thereby increases, 2) energy required for a refill (such as for precooling or a pressure overfill) is reduced, 3) time is saved, and 4) overall customer satisfaction increases.